The invention relates to protection circuitry for protecting circuits against transients such as electrostatic discharge (ESD). In particular, the invention relates to circuitry for protecting analog bipolar circuits, including BJT and BICMOS based circuits against voltage transients.
Analog circuits typically display sensitivity to excessive voltage levels. Transients, such as ESD can cause the voltage handling capabilities of the analog circuit to be exceeded, resulting in damage to the analog circuit. Clamps have been devised to shunt current to ground during excessive voltage peaks.
Typical protection clamps employ avalanche diodes such as zener diodes 50 to provide the bias voltage for the base of a subsequent power bipolar junction transistor (BJT) 52 as shown in FIG. 5. Since separate, stable avalanche diodes in a given technology are difficult to realize, zener diodes are typically implemented as punch-through, or reverse coupled BJT""s, as shown in FIG. 6. FIG. 6 shows a prior art clamp comprising a reverse coupled BJT 600 that acts as an avalanche diode. The BJT 600 has a defined reverse breakdown. A voltage pulse supplied to the input 602 that exceeds the breakdown voltage of the BJT 600, causes an avalanche effect in BJT 600 and results in current flow from the emitter 604 to the collector 606 when the reverse breakdown voltage of the BJT 600 is exceeded. The current drives the base 608 of the power BJT 610 and switches BJT 610 on by raising the voltage across the resistor 612, thus biasing the base 608 to switch on BJT 610. Once the BJT 610 switches on, collector/emitter current is shunted to ground. To reach a higher protection voltage, several zener diodes or reverse coupled BJT""s may be connected in series. For example, in a five volt power supply circuit where the BJT 610 is a 10 volt BJT, two 3.5 volt zener diodes could be used to replace BJT 600. Until the BJT 610 switches on, the voltage 602 across the clamp, which is also the voltage applied to the protected circuit, continues to increase during a voltage peak such as a human body discharge. Such a discharge may typically be 120-125% percent of the final holding voltage.
One of the difficulties encountered in designing such protection circuitry is that the specifications for these clamps have to fit within a relatively small design window that takes into account the breakdown voltage of the circuit being protected. Thus, the clamp must be designed so as to be activated below the breakdown voltage of the circuit that is to be protected. At the same time, the design window is limited by the latchup phenomenon. To ensure that the clamp is not conducting under normal operating conditions, the latchup voltage must exceed the normal operating voltage of the protected circuit.
Since the breakdown voltage of a particular punch-through structure in the BJT clamp is determined by the particular technology process, prior art devices have difficulty complying with the small ESD protection window especially for high-voltage circuits, where maximum breakdown voltage can be only 10% higher than the operating voltage.
The present invention provides a protection clamp against transient conditions, that provides an adjustable ESD protection window. The invention uses the ability of a reverse coupled BJT to provide different breakdown voltage characteristics at different base current multiplication conditions. Base current multiplication is controlled by providing a resistor between the collector and the base of the reverse-coupled BJT.
According to the invention, there is provided an overvoltage protection circuit for protecting an input of an analog bipolar circuit comprising a first bipolar junction transistor having a base, a collector, and at least one emitter, connected between an input of the analog bipolar circuit and ground, a reverse-coupled bipolar junction transistor having a base, a collector, and at least one emitter, wherein the emitter is connected to the input, and the collector is connected to the base of the first bipolar junction transistor, and a resistor connected between the base and the collector of the reverse coupled bipolar junction transistor. The resistor is connected to the reverse-coupled transistor so as to increase base current multiplication in the reverse-coupled transistor.
Further, according to the invention, there is provided a method of improving an overvoltage protection circuit for an input to a protected circuit, wherein the overvoltage protection circuit includes a first transistor for shunting current to ground, and a reverse-coupled transistor connected to the first transistor to switch on the first transistor, the method comprising the step of lowering the reverse-breakdown voltage of the reverse-coupled transistor when a voltage transient occurs by increasing base current multiplication.